Harmful gas leakage accidents often occur in chemical plants. Harmful gas leakage may cause the loss of lives or serious accidents such as a fire. In industrial fields, detectors are almost necessarily used to detect a leakage of harmful gas. Furthermore, detectors may be used in various other fields such as nuclear, biological and chemical reconnaissance, measurement of air pollution, detection of harmful materials, etc. Accordingly, the demand for detectors is increasing. Detectors are being developed to satisfy the requirements of portability, ease of use, high sensitivity, and high optionality.